1. Field of the Invention
The present invention relates to methods and systems for detecting tornadoes by means of their electromagnetic signatures, and more particularly, to such methods and systems which are capable of distinguishing between electromagnetic signals caused by tornadoes per se and other electromagnetic signals which are either manmade or associated with common lightning.
2. Description of the Related Prior Art
Tornado detection systems which operate based on the identification of drastic changes in ambient atmospheric pressure, and more particularly rapid reductions in atmospheric pressure, have been previously proposed. One such system is disclosed, for example, by U.S. Pat. No. 3,029,422 to Wilson. Such systems have been found to have certain shortcomings, including a preponderance of false alarms, since low atmospheric pressures, and even the sudden onset of such low atmospheric pressures, can be associated not only with tornadoes, but also with other storm systems of varying severity. As a result, merely detecting the presence or onset of low atmospheric pressures is not a reliable indicator of tornadoes.
On the other hand, a system disclosed in U.S. Pat. No. 3,646,540 to Cooper makes use of the well known phenomenon that approaching tornadoes can cause the appearance of "static" on the screen of a TV monitor, particularly when the monitor is tuned to a lower part (typically channel 2 or about 55 MHz) of the broadcast frequency range. Moreover, such a system is used in conjunction with an aneroid switch which closes when ambient atmospheric pressure falls below a predetermined limit. However, this system also has disadvantages, including the need to employ a miniature TV screen, a light detector and an aneroid switch, making the system expensive and bulky. Furthermore, this system also suffers from an unacceptable rate of false alarms when used in areas where manmade electromagnetic signals from TV stations are prevalent. Although such problems can be alleviated to some extent by fine tuning and frequent calibration of the system to account for the presence of manmade signals which can occur within the same spectral range as that used for detecting tornadoes, as a practical matter, such calibration has to be carried out at each site for which the warning device is intended, and cannot be done during production of the system at the factory.
Bacon et al. in U.S. Pat. No. 3,810,137 propose an alarm system employing a receiver which is tuned to a single frequency in a range of between 20 MHz to 60 MHz, and preferably to 53.25 MHz. The system is additionally coupled to an audio frequency detector for detecting and amplifying the amplitude modulations of a carrier signal in the range of 5 to 2000 Hz which are hopefully caused only by the presence of an oncoming tornado. However, manmade signals within the disclosed frequency range (i.e. TV channel 2 or 55 MHz) can also include amplitude modulations, making calibration of the system for given areas difficult. Essentially, a threshold for the system must be set at which the system will activate an alarm, however, such a calibration can be carried out accurately only for certain limited locations where manmade signals are present only intermittently and not at a constant intensity. Furthermore, in order to be useful, such calibration and threshold settings must be carried out in the presence of an approaching tornado, so as not to result in false alarms (when the threshold is set too low) or result in missing actual threatening tornadoes (when the threshold is set too high). Furthermore, such calibration must take into account the local nature of manmade RF signals, and thus this system cannot be used as a mobile alarm system. The system also suffers from false alarms generated due to normal lightning associated with approaching storms which may not necessarily generate tornadoes, but which can nevertheless activate the alarm too frequently.
Finally, Kennedy et al. in U.S. Pat. No. 4,812,825 describe a tornado alarm system in which an RF receiver is designed to receive an amplitude modulated signal within a broad range of 1 MHz to 100 MHz. Such a receiver is used in conjunction with a wide band amplifier and an integrator which activates a tornado alarm if and only if the amplitude modulation, generated by an approaching tornado, exceeds a minimum threshold for a minimum period not shorter than 3 seconds and, in most instances, between 10 to 25 seconds. The long term integration is designed to differentiate between the continuous electromagnetic noise generated by a tornado and intermittent electromagnetic noises generated by less severe storms, particularly lightning events. However, such long integration periods severely cut the time available for seeking shelter. Furthermore, this system, as with the system described in U.S. Pat. No. 3,810,137, requires the setting of threshold levels, through an automatic gain control, to prevent activation by manmade signals, and these thresholds are necessarily location dependent as well.
To the extent that U.S. Pat. Nos. 3,646,540, 3,810,137 and 4,812,825 demonstrate the well known fact that tornadoes generate electromagnetic noise, and describe means for detecting such noise within certain regions of the electromagnetic spectrum, the disclosures of these patents are hereby expressly incorporated into the present disclosure by reference.